1. Field of the Invention
The present invention is directed generally to a method and apparatus for controlling the distribution of electric energy, and more specifically, to a method and apparatus for adapting coordination of protective measures provided to an electric-energy distribution system as a function of at least a sensed weather condition.
2. Description of Related Art
Traditionally, electric energy has been transmitted away from generation facilities to be distributed to residential, industrial and commercial structures by a network of conducting wires commonly referred to as power lines. The network of power lines forms a portion of what is commonly referred to as a power grid. The power grid also includes the upstream generation facilities and substations disposed electrically between the generation facilities and the end users of the electric energy. Electric energy is conducted between the generation facilities and the substations by the power lines, which also distribute the electric energy from the substations to the various types of structures.
Electric energy, also commonly referred to as electricity, is produced at the generation facilities at relatively low voltages and then has its voltage stepped up by the power station transformer to high transmission voltages. High transmission voltages are often several hundred kilovolts, usually topping out at about 765 kV to conduct electric energy over long transmission distances. The high transmission voltages are necessary to minimize losses due to the impedance of the power lines as those lines conduct the electric energy over long distances to substations. At the substations, the high voltages are stepped back down to a suitable level, usually between 2.4 kV and 33 kV, for transmitting the electric energy the remaining, relatively short distances over a distribution grid to the various structures occupied by the end users of the electricity. Distribution transformers provided near the structures of the end users perform the final voltage step down to deliver the desired voltage to the end users, and this finally-stepped-down electric energy is then conducted by consumer lines to the structures occupied by the end users.
In the power grid described above, the generation facilities are considered to be “upstream,” meaning that they act as the source from which the electric energy flows. Likewise, the electric energy flows “downstream” from the generation facilities to the substations, and eventually, on to the end users. One, or a relatively small number of high-voltage transmission lines can conduct the high-voltage electric energy downstream from a generation facility to a substation. At each substation a larger number of distribution lines carrying electric energy at a voltage that is lower than the electric energy carried by the high-voltage transmission lines branch off from the high-voltage transmission line. The distribution lines can run parallel to public roadways and conduct electric energy from the substations to the various structures such as residential homes. A service line then conducts electric energy at a voltage suitable for each particular structure from a distribution line to the structure, and there can be many structures that receive electric energy from a given distribution line. Thus, many distribution lines can branch out from a small number of high-voltage transmission lines at a substation, and a large number of service lines can branch off of each distribution line at structures occupied by the end users of the electric energy.
An electric utility supplying electricity to a region divides the power grid into zones to minimize the effect of a fault condition on the power grid as a whole. Each zone is separated from others by one or more primary electrical isolation devices such as feeder breakers that are located at a substation and are designed to isolate a zone in which a fault condition has occurred. For example, a feeder breaker can disconnect an entire distribution line from the power grid, thereby disrupting the flow of electricity to that distribution line. Accordingly, all structures that are supplied with electricity from that distribution line will experience a loss of electricity until it is determined that the detected fault condition no longer exists.
Additionally, secondary isolation devices such as fuses are distributed within each zone throughout the power grid to further isolate a fault condition. By isolating the fault to a particular zone, or even to a particular portion of the zone, the number of structures to which the delivery of electricity is interrupted is minimized.
Previous systems for controlling the interaction of protective measures provided to the power grid have attempted to minimize the number of structures that lose electricity service while still isolating the fault condition. Such conventional power grid protection systems, however, have their shortcomings. One conventional protection system employs a fuse blowing philosophy. According to such a philosophy, the substation feeder breaker is properly coordinated with the downstream fuses. In the event of a fault condition, a fuse will clear any downstream fault within its rating instead of the feeder breaker.
The fuse blowing philosophy is effective in isolating the fault condition while minimizing the number of electricity customers that lose electric service. Structures upstream of the blown fuse and serviced by the distribution line will not experience a disruption of electricity service. However, the problem with this philosophy is that the electricity service to customers fed by the distribution line and located downstream of the fuse that is blown is permanently interrupted, even if the fault condition is only a temporary fault. Further, the electricity service remains interrupted until the utility servicing that distribution line can manually replace the blown fuse, an expensive and time consuming process, particularly if there are widespread reports of blown fuses.
Other conventional power grid protection systems have employed a fuse saving philosophy. According to this philosophy, the first trip of the substation feeder breaker is intentionally miscoordinated so that the breaker operates faster than the fuse to clear a fault downstream of the fuse. The speed with which the first trip of the feeder breaker occurs prevents the fuse from blowing, and following a short delay, the feeder breaker is reset. If the fault condition is still present when the breaker feeder is reset, any further trips of the feeder breaker will be intentionally is slower than operation of the fuse so that the fault condition is isolated when the fuse blows. However, the fuse saving philosophy causes all customers serviced by the distribution line protected by the feeder breaker experience a momentary interruption for all faults, instead of just the customers downstream of the fuse.
Both the fuse blowing and the fuse saving philosophies are rigid, operating under the same principles regardless of the condition that could possibly have caused the fault condition. The fuse blowing system always results in a permanent interruption of electricity service upon the detection of a fault condition, even if that fault condition is temporary. And although the fuse saving system can minimize permanent interruptions of electricity service due to temporary faults, it always results in at least a brief interruption of electricity service to all customers serviced by the entire distribution line protected by the feeder breaker. Neither system takes into consideration the circumstances possibly leading to the fault condition to optimize performance of the power grid under different conditions.
Accordingly, there is a need in the art for a power grid protection system and method that protects the power grid based at least in part on a possible cause of a fault condition. The system and method can offer protection of the power grid automatically and without operator intervention based at least in part on a sensed weather condition. Further, the system and method can optionally allow for manual selection of the power grid's protection.